This invention relates to test equipment for measuring the performance of electrical and electronic devices, and more specifically to current-voltage, or I-V curve tracers. An illustrative embodiment described herein is particularly suited to measure the I-V relationship of high-power photovoltaic arrays.
Curve tracers are useful for designing, diagnosing, monitoring, testing or measuring the performance of electrical devices, such as diodes, transistors, batteries, electrical circuits, and the like. Such curve tracers depict a relationship typically between a current, I, and a voltage, V, of the device or component being monitored under various load or power conditions. Relationships between parameters other than voltage and current may also be depicted (e.g. between two currents, or between a voltage and a power or, between other functions of voltage and current); herein the term "I-V" will be understood not to be limited to voltage and current relationships, but to include relationships between or among various other parameters as well. The information derived from these I-V curves is useful in characterizing the operation of the device, and in some instances determining whether an abnormal or faulty condition exists.
The device being tested may be active or passive; that is, it may generate energy or dissipate energy. If the device is active and generates energy, the curve tracer must in turn receive the energy produced by the device during the test, in which case the curve tracer typically includes an energy sink for receiving, storing and/or dissipating the energy. On the other hand, if the device is passive and dissipates energy, the curve tracer typically includes means for generating and supplying energy to the device to enable its I-V relationship to be measured. For both active and passive devices, however, measurements of the current and the voltage are made at various load levels of the device under test.
Photovoltaic solar cells, e.g. diodes, are active devices that generate energy. Their popularity has grown in recent years due to a recognized need to develop alternative energy sources. As a result, there now is a need for a general purpose portable test instrument for monitoring in the field a wide variety of photovoltaic solar cell arrays, and for diagnosing faults should such arrays become wholly or partly inoperative. Examples of inoperative conditions include degradation of the semiconductor material from which the solar cells are fabricated and reduction of the transmissivity of the protective transparent coating that covers the cells. Comparative analysis of the shapes of I-V curves taken at different times can reveal such changed conditions including the extent and nature of the faults.
In a photovoltaic array, the I-V curve is conventionally generated by connecting the output terminals of the array to the input terminals of a load. In general, an I-V curve can be swept out by changing some parameter of the load and recording the resulting voltage across the array and the current flowing through the array (i.e. the voltage and current at the array/load interconnection point).
If the load is an adjustable voltage sink, an I-V curve can be measured by sweeping the load voltage as a function of time while recording the resulting current; this is illustrative of conventional approaches for determining I-V curves, which are based on what can be called "forced sweep". In such forced sweep systems, the load is changed independently of the voltage and current at the point of interconnection between array and load.
Forced sweep is contrasted with "feedback sweep" in which the sweeping action of the load is controlled completely and exclusively as a function of the voltage and current at the array/load interconnection point.
An important subclass of feedback sweep is "natural sweep". Once initiated, the time course of a natural sweep depends solely on properties of the device under test (e.g. the array) and on properties of the load. With natural sweep, the "control" of the load is implicit in the load component itself. For example, in a test using a capacitive load, the energy storage capability of the load capacitor results in effectively changing the load. This is contrasted with a test using a resistive load, in which control circuits are used to vary the load.
A system using natural sweep can be much simpler than one using forced sweep. Forced sweep requires a variable load device and control circuitry to force the load to vary according to some external control or as a specified function of time. With natural sweep, a fixed value load device may be used, and sweep control circuitry to force the load to vary is entirely unnecessary.
In choosing an approach to determining I-V curves, particularly when high power levels are involved, it is also important to decide whether the emphasis will be on the curve shape as a whole or on continuously monitoring one point of the I-V curve. To continuously monitor one point requires that the curve tracer be able to continuously handle, i.e. either as a source or as a sink, the power of the chosen point in the I-V curve. Depending on the application, the resulting power could be ones, tens or even hundreds of thousands of watts. Handling this power requires large heat sinks and cooling fans. On the other hand, if determining curve shape is the principle concern, it is possible to make a transient measurement. By sweeping rapidly through the I-V curve, thereby spending only a short amount of time at each point, the power handling requirements can be drastically reduced. The reduction factor is approximately the time from one measurement to the next over the duration of a measurement. If, for example, it takes 0.01 seconds to sweep through a curve and one waits only 10 seconds between curves, the average power is reduced by a factor of 1000 over that required by the continuous monitoring approach. Thus implementing the transient sweeping approach avoids the need for large heat sinks and cooling fans.
As will be seen below, there are advantages to combining natural sweeping with the transient measurement approach, particularly for photovoltaic arrays. For photovoltaic arrays, a reactive load, such as a capacitor or inductor, provides the basis for achieving both of these objectives in a simple manner. With a capacitive load the sweep of the I-V curve begins at short circuit current and proceeds to open circuit voltage. An inductive load causes a sweep in the opposite direction, i.e. the curve starts at open circuit voltage and proceeds to short circuit current. A further advantage of the capacitive load is that it is easier to switch it onto and off of the array.
In natural sweep systems, generally neither current nor voltage are linear functions of time. Moreover, for a fixed quantity of load impedance, the total sweep time depends upon the magnitude of the open-circuit voltage and the short-circuit current of the array. Thus, in order to produce an I-V curve having a good distribution of sample points along the curve, the amount of load impedance should be selected so that the total sweep time is large enough to permit a sufficient number of samples to be taken, and control circuitry should be employed to assure that samples are taken at evenly distributed points along the I-V curve.
One prior system that uses a capacitive load in measuring the current-voltage relationship of a photovoltaic array was reported by Ronald C. Cull et al. in connection with work on a solar energy project sponsored by the U.S. Department of Energy at the NASA/Lewis Research Center in Cleveland, Ohio. They describe the system in an IEEE paper entitled "The DOE/LeRC Photovoltaic Systems Test Facility" published 1978. In the NASA/Lewis system, before conducting the test, steady-state short-circuit current flows through a shunt switch connected across the array in parallel with a capacitive load. To start sweeping, the shunt switch is opened; the capacitor charges, approaching open-circuit voltage; and the array current diminishes, approaching zero as the capacitor charges. After opening of the shunt switch, a predetermined number of measurements of the current through and voltage across the array are taken at fixed time intervals. An auxilliary current source may also be employed to charge the capacitor simultaneous with the array, and further to continue to charge the capacitor above the open-circuit voltage of the array.
According to a draft report obtained July 27, 1981 entitled "High-Speed Computerized Data Acquisition of Photovoltaic V-I Characteristics" by Cull et al., to provide different sweep times for different magnitudes of array voltages, the NASA/Lewis system makes use of a relatively large number of parallel connected capacitors, about 66,760 microfarads in total capacitance, and a plurality of switches and conductors to couple across the array a certain amount of capacitance. It is necessary in the NASA/Lewis system to finely adjust the value of the capacitive load in order to obtain satisfactory I-V curve measurements. Accordingly the amount of capacitance used for the sweep is switched into the system in incremental steps of 50 microfarads and the system's circuit arrangement is loaded with bulky and expensive switches and conductors to attain the desired capacitance. The system is thus not very suitable to be conveniently carried by hand to typically remote locations of photovoltaic arrays. Additionally, the NASA/Lewis system momentarily closes a shunt switch across the array to force it to short-circuit conditions. In the event that the shunt switch is closed inadvertently with charge on the capacitor, the shunt switch is destroyed due to the current surge from the capacitor, presenting a hazard to operating personnel.
Furthermore, the NASA/Lewis system does not provide any means for assuring that the voltage and current measurements at the output terminals of the device under test coincide with the occurrence and rate of natural sweeping so as to produce a sufficient number and even distribution of sample points along an I-V curve for a variety of electrical devices. We define this assurance as "capturing" the measurements in transit.
Another prior photovoltaic I-V curve tracer is described in U.S. Pat. No. 4,129,823. The system described uses a forced rather than natural sweep, and samples are taken at equal increments of time. This system varies the value of a resistive load as a nonlinear function of time; thus the samples which are taken at equal increments of time are distributed more evenly on the I-V curve than they would be if the value of the resistive load was varied linearly with time. The nonlinear characteristic of a field effect transistor is used to change the array load resistance in very fine steps at the low resistance region of the sweep, and in large steps at the higher resistance region of the sweep.
Other known curve tracers using forced sweep are described in U.S. Pat. No. 4,184,111 which employs a resistive load and a variable voltage source, and in U.S. Pat. No. 4,163,194 which varys a resistive load such that the voltage across the array either increases linearly with time or varys in response to a user control.